Liquid phase sintered cemented carbide materials, such as tungsten carbide using a cobalt binder (WC-Co), are well known for their high hardness and wear and erosion resistance. These properties have made it a material of choice for mining, drilling, and other industrial applications that require strong and wear resistant materials. Cemented tungsten carbide's properties have made it the dominant material used as cutting inserts and insert compacts in rock (tri-cone) bits and as substrate bodies for other types of cutters, such as superabrasive (generally polycrystalline diamond compact, or “PDC”) shear-type cutters employed for subterranean drilling as well as for machining and other industrial purposes. However, conventional liquid phase sintered carbide materials such as cemented tungsten carbide also exhibit undesirably low toughness and ductility.
Conventional fabrication of cemented tungsten carbide is effected by way of a liquid phase sintering process. To elaborate, tungsten carbide powder is typically mixed with cobalt powder binder material and fugitive binder such as paraffin wax, and formed into a desired shape. This shaped material is then subsequently heated to a temperature sufficient to remove the fugitive binder and then further heated to a temperature sufficient to melt the cobalt and effectively “sinter” the material. The resulting components may also be subjected to pressure, either during or after the sintering operation to achieve full densification. The sintered material comprises tungsten carbide particulates surrounded by a solidified cobalt phase.
As alluded to above, in conventional liquid phase sintered tungsten carbide materials, as with many materials, fracture toughness is generally inversely proportional to hardness, while wear resistance is generally directly proportional to hardness. Although improvements in the fracture toughness of cemented tungsten carbide materials have been made over time, this parameter is still a limiting factor in many industrial applications where the cemented tungsten carbide structures are subjected to high loads during use. The material properties of cemented tungsten carbide can be adjusted to a certain degree by controlling the amount of cobalt binder, the carbon content, and the tungsten carbide grain size distribution. However, the bulk of the advancements using these conventional metallurgical techniques have largely been realized. U.S. Pat. No. 5,880,382 to Fang et al. attempts to solve some of the limitations of conventional WC-Co materials but uses expensive double cemented carbides.
Another drawback to conventional cemented tungsten carbide materials is the limitation of using cobalt as the binder. About forty-five percent of the world's primary cobalt production is located in politically unstable regions, rendering supplies unreliable and requiring manufacturers to stockpile the material against potential shortfalls. Also, about fifteen percent of the world's annual primary cobalt market is used in the manufacture of cemented tungsten carbide materials. A large percentage of the cobalt supply is used in the production of superalloys used in aircraft engines, a relatively price-insensitive application which maintains fairly robust levels of cobalt prices. These factors contribute to the high cost of cobalt and its erratic price fluctuations.
Cobalt has also been implicated as a contributor to heat checking when used as inserts in rolling cutter bits as well as in tungsten carbide substrates for cutters or cutting elements using superabrasive tables, commonly termed polycrystalline diamond compact (PDC) cutters. Heat checking, or thermal fatigue, is a phenomenon where the cemented tungsten carbide in either application rubs a formation, usually resulting in significant wear, and the development of fractures on the worn surface. It is currently believed that thermal cycling caused by frictional heating of the cemented tungsten carbide as it comes in contact with the formation, combined with rapid cooling as the drilling fluid contacts the tungsten carbide, may cause or aggravate the tendency toward heat checking. The large difference in coefficient of thermal expansion (CTE) between the cobalt binder and the tungsten carbide phase is thought to substantially contribute to heat checking fracture. Another disadvantage of conventional WC—Co materials is that they are not heat treatable and cannot be surface case hardened in such a manner that is possible with many steels.
Non-cobalt-based binder materials such as iron-based and nickel-based alloys have long been sought as alternatives. U.S. Pat. No. 3,384,465 to Humenik, Jr. et al. and U.S. Pat. No. 4,556,424 to Viswanadham disclose such materials. However, problems due to the formation of undesirable brittle carbide phases developed during liquid phase sintering causing deleterious material properties, such as low fracture toughness, have deterred the use of iron-based and some nickel-based binders. Therefore, it would be desirable to produce a carbide material whose cementing phase exhibits, to at least a substantial degree or extent, the original mechanical characteristics (e.g., toughness, hardness, strength), thermo-mechanical characteristics (e.g., thermal conductivity, CTE), magnetic properties (e.g., ferromagnetism), chemical characteristics (e.g., corrosion resistance, oxidation resistance), or other characteristics exhibited by the binder material, in a macrostructural state. It is further desirable that the binder be heat treatable for improvement of strength and fracture toughness and to enable the tailoring of such properties. Further, the cemented carbide material should be capable of being surface case hardened, such as through carburizing or nitriding. In addition, the reduction or elimination of deleterious carbide phases within the cemented carbide material is desired. The present invention fulfills these and other long felt needs in the art.